This invention relates to the field of tone synthesizers and more particularly to synthesizers such as are utilized in communications devices which must provide predetermined tone sets with great accuracy.
Tone synthesis is known to be achievable by programmed dividers coupled to a fixed signal source such as a crystal oscillator. Since such dividers in present day equipment are typically included in an IC, there is little flexibility to improve the accuracy of the tone set frequencies. Certain equipment requires the capability of changing from one set of tones to another; e.g. from ZVEI (Zentral-Verband der Electro-industrie) to CCIR (International Radio Consultative Committee or Comite' Consultatif International Radio) standard tones. While the CCIR standard calls for an accuracy of plus or minus 8 Hz, in some instances the required tones of the respective standards are as close as 5 Hz and have been distinguished at decoders solely by tone length which might be as long as 85 msec. for a CCIR tone of 1275 Hz or as short as 90 msec. for ZVEI tone of 1270 Hz. (ZVEI tone length is 70 msec.+-.15 msec and CCIR tone length is 100 msec.+-.10 msec.). Since the difference between the two tone lengths could be so slight, frequency accuracy becomes a critical requirement. Also, a more accurate encoder would reduce the system cost since narrow bandwidth (lower priced) decoders can be used.
Microprocessors have been used to provide tone synthesis and can overcome many of the problems of previous synthesizers. However, since the tones are produced by the addition of many increments, accuracy is limited by the size of the increment, which is a function of the oscillator frequency. Thus the accuracy is determined and limited by the frequency which is limited by the delay times of the specific circuitry.